Recent clinical studies have defined the presence of individuals with natural immunity to infection with filarial parasites. Some infected individuals may remain microfilaremic over long periods of time, without developing overt lymphatic pathology, but others develop such pathology that can develop either in the setting of persistent microfilaremia or after microfilaremia has cleared. PCR-based amplifications of parasite DNA in skin snips from patients with onchocerciasis has increased diagnostic sensitivity and specificity, and the assays have been configured into an ELISA format that is field applicable. Similar efforts are underway to detect parasite DNA in lymphatic filariasis, but sensitivity is not yet superior to diagnosis by parasitologic or antigen-detection means. A treatment trial comparing two or seven 1-week courses of DEC for lymphatic filariasis given over 15 months showed equivalent therapeutic effects but, most importantly, showed for the first time that circulating filarial antigen is an excellent and readable marker of active bancroftian filarial infection. Other studies showed that single yearly doses of ivermectin or DEC are extremely effective in reducing microfilaremia in bancroftian filariasis, and either drug alone (or together) would be appropriate for control programs. In contrast, even three 3-week courses of DEC could cure only about two thirds of expatriates with loiasis; more effective chemotherapy is needed.